1. Field of the Invention
The present invention relates to extendible and retractible mast systems, and more particularly to such a system utilizing a plurality of telescoping nestable tubes useful for supporting payloads such as antennas, electronic sensors/transmitters, cameras and the like.
2. Description of the Prior Art
The use of communications and various electronic and optical equipment in hostile military environments often requires erecting antennas and the like as high as 20 meters or more above ground in order to facilitate the transmission and reception of signals to and from distant locations. With portable tactical military systems, it is often essential that such an antenna be elevated from and quickly retracted to a stowed position on a transporting vehicle with a minimum of personnel and in all types of hostile environments. These requirements are particularly difficult during winter months. At below freezing temperatures, ice formations tend to lock the extended mast sections together and to block or resist retraction of the mast to a stowed/nested position. Ice accretion not only impedes retraction and disassembly but also tends to unduly load and possibly damage any exposed elements or parts of the mast retracting mechanism. In addition to ice accumulation winter winds increase the cantilevered forces exerted on an extended loaded mast. Furthermore, exposure of the elevating and retracting mechanism to harsh weather conditions causes corrosion and deterioration of exposed parts so as to hamper their operation and require frequent repair or replacement thereof.
Early portable mast systems consisted of a plurality of separate identical mast sections mechanically coupled together and raised into and secured usually in a vertical operating position with guy wires. This required the handling of many separate parts and also required considerable time to erect and dismantle the mast. Consequently for dynamic tactical military conditions, such systems were useless or nearly so. Other more sophisticated prior mast systems employed nested tubes that were extended and retracted by pneumatic or hydraulic means. Not only were these systems complex and very expensive, but they were especially vulnerable to disablement by bullets or shrapnel in hostile military environments. A single projectile penetrating an extended mast could easily have caused decompression of the pneumatic or hydraulic chamber or alternatively caused the mast to be locked in an extended position through deformation of the metallic tube(s).
Other drawbacks of pneumatic masts are they are very heavy which adversely affects portability, especially in the field., they have no positive means of retraction., large space is required to store the air tanks; and loss of power results in immediate retraction due to decompression.
Present use of extruded cylindrical aluminum tubes has several disadvantages: it is very difficult to extrude tubes having diameters in excess of 11 inches, thereby limiting the use of larger tubes., tolerances are difficult to hold tightly, resulting in costly rejects and fit-match procedures; circular tubes require keys and keyways to prevent rotation between tubes, and aluminum keys are readily subject to wear which results in excessive play between tubes.
This invention is directed to a mast system which overcomes these shortcomings and disadvantages.